Composite building units

Figure 2.1 Common ways of representing the d4r composite building unit formed from eight tetrahedra in the shape of a cube. The standard view (a) represents T and O atoms as spheres. Tetrahedral representations...

Figure 2.2 Some examples of polyhedral composite building units with their corresponding pore symbols and common names. The nodes are tetrahedrally coordinated atoms such as Si or Al. Bridging oxygen atoms have been left out for clarity.

Figure 2.3 Some examples of chain composite building units, (a) Single chain (zig-zag) and (b) double chain (double zig-zag) with periodicities of two. (c) Single chain (crankshaft) and (d) double chain (double crankshaft) with periodicities of four, (e) Complex chain (designated mod by Smith...

Introductory Chapter 1 provides a historical overview of molecular sieve materials. Chapter 2 covers the definition of a zeolite and describes their basic and composite building units and how they are linked in zeolite frameworks. It defines pores, channels, cages and cavities and it gives references for finding detailed... 

Fig. 31. Merlinoite, zeolite W. (a) Composite building units, (b) channels [70B1]. (d) The polyhedra representation of the framework zeolite W with the unit cell outlined [96B2]. (cont.)...

Figure 16.1 A d4r composite building unit occluding a fluoride anion (white sphere)...

Finally, the ionothermal synthesis route developed since 2004 by Morris et al. [187] was not successful for the silica-based materials but the ionic liquids themselves can act as classical OSDAs in aqueous media. Thus, by using the fluoride route, Zones et al. synthesized pure silica zeolites of TON, ITW and MTT topologies using the OSDAs l,3-dimethyl-37/-imidazol-l-ium, l,2,3-trimethyl-3//-imidazol-l-ium and 1,3-diisopro-pyl-3//-imidazol-l-ium, respectively [12]. Then, in 2009, we reported the synthesis of IM-16 (UOS) a germanosilicate prepared with the ionic liquid 3-ethyl-l-methyl-3//-imidazol-l-ium as OSD A in aqueous media. This microporous material possesses a new topology built from d4r and mtw composite building units [165]. The ionothermal synthesis route, up to now, was only successful in the formation of new phosphate-based microporous materials, as mentioned in the next paragraph. 

Recently, Okubo and coworkers built a working hypothesis in seed-assisted synthesis without organic templates, which may provide a new idea for synthesis and broaden framework types of zeolites [40]. By analyzing the examples of the seed-assisted and organic-template-free synthesis of zeolites in detail, they concluded that the composite building units that both the frameworks of zeolite seeds and the crystallized zeolites contained were the key to the organic-template-free synthesis. 

A survey of cages D = Qi) as type characteristics is added in Appendix 1. The polyhedrons are arranged in increasing number of T atoms ( ) defining the smallest faces of the polyhedron and in increasing nirmber of feces (21m, ). Appendix 2 and Appendix 3 srrmmarizes those channels D = 1) and cavities iP> 1) that appear in more than one fiamework t)q)e. An extensive compilation of polyhedral units in zeolites is published by Smith f A list of those polyhedral imits (or Composite Building Units) that are foimd in at least two different fiamework types is also published in the new edition of the Atlas ... 

The initial transition of dissolved silicate molecules into solid nanoparticles is perhaps the least explored step in the synthesis of zeolites. One impediment to understanding this mysterious step is the poorly elucidated molecular composition of dissolved particles. The major mechanistic ideas for the formation of zeolites approach these structures differently i) many researchers believe that secondary building units (SBU) must be present to form initial nanoslabs [1,2] ii) some others prioritize the role of monomers to feed artificially introduced crystal nuclei or assume that even these nuclei form via appropriate aggregation of monomers [3] iii) silicate solutions are also frequently viewed as random mixtures of various siloxane polymers which condense first into an irregular gel configuration which can rearrange subsequently into a desired crystal nucleus at appropriate conditions [4,5],... 

The results of the discussion on the phenomenological thermodynamics of crystals can be summarized as follows. One can define chemical potentials, /jk, for components k (Eqn. (2.4)), for building units (Eqn. (2.11)), and for structure elements (Eqn, (2.31)). The lattice construction requires the introduction of structural units , which are the vacancies V,. Electroneutrality in a crystal composed of charged SE s requires the introduction of the electrical unit, e. The composition of an n component crystal is fixed by n- 1) independent mole fractions, Nk, of chemical components. (n-1) is also the number of conditions for the definition of the component potentials juk, as seen from Eqn. (2.4). For building units, we have (n — 1) independent composition variables and n-(K- 1) equilibria between sublattices x, so that the number of conditions is n-K-1, as required by the definition of the building element potential uk(Xy For structure elements, the actual number of constraints is larger than the number of constraints required by Eqn. (2.18), which defines nk(x.y This circumstance is responsible for the introduction of the concept of virtual chemical potentials of SE s. 

Component Composition Building blocks (repeat unit) Location... 

The ionic composition of dilute sodium silicate is a very complex problem involving Na20/SiC>2 ratio, water content, and even trace impurities. Equilibration seems to be very slow at ordinary temperatures. As shown in Figure 5, Harris, et.al, were able to identify a wide variety of structures in potassium silicate solutions which bear a striking resemblence to the secondary building units proposed for zeolite... 

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